The proper way to do it is 3d8 + 9. This gives you a full range of results and a "central tendancy" that makes it more likely to give mid-range values (closer to 21-24) and less likely to give end-point values (12-14, 31-33).

(1d8+3) x3 is a shortcut but it will remove the bell-curve and you're just as likely to get any particular value as any other. All given values will be "3 between" and it increases the chance of extreme highs and lows.